1. Field of the Invention
The invention relates to electrophoretic displays, in particular electrophoretic displays in which their driving torque is originated from a ferroelectric coupling.
2. Background of the Invention
Memory type display devices are attracting intense research and product consideration from the beginning of flat panel display industry due to the advantage in power consumption and readability under sun light ambient luminance condition. Some liquid crystal displays are being used for this purpose with their memory function. In past several years, some types of electrophoretic displays are widely in use, particularly in as so-called e-reader displays.
A memory function based reflective display technology is suitable for display devices specifically for character displays just as paper like image. Reflective nature of display image is also very suitable for replacement of paper media that is strongly required in terms of saving paper resources as well as electronic energy power saving point of view. A replacement of paper media point of view, it is quite natural that so-called an e-paper device has a display function of only still image. A memory function of those e-paper types of display modules saves their power consumption significantly thanks to their memory function. This significant power saving characteristic property is also good match with replacement of paper media.
On the other hand, a paper like electronic display device is strongly expected to have a function of so-called full color. It is quite natural requirement for an electronic paper type display screen to show full color image shown in a paper media. A full color characteristic property is one of the challenges for most of memory types or electrophoretic display devices. In principle, a memory function by display device medium does not show straightforward compatibility with full color display function. In most of memory type display technologies are based on bistability of display medium itself. Consequently, multiple screen luminance level display technology and memory state display technology have a fundamental difference in their principle of display functions. A typical memory function of display device itself uses so-called bi-stability, or alternative of two stable states. Therefore, memory function and gray scale reproducing capability based on multiple state stable states are incompatible. Regardless fundamental issue of multiple state stability, it is quite natural full color image on an e-reader is required. In order to obtain full-color function with state-of-the-arts technologies, a micro-color filter in conjunction with sub-pixels technology is widely used. This technology is based on spatial resolution limitation of human eyes. This state-of-the-arts technology is good enough to be used for so-called e-reader application based on bi-stability type display technology as long as it is applied to still images. However, unlike back light type display devices, reflective display's color recognition function is entirely dependent on ambient light luminance and major wavelength. Moreover, use of sub-pixel reduces image resolution at least to one third compared to the original image resolution. Therefore, for most of reflective displays, obtaining reasonable color purity level with good enough luminance display needs entirely new concept to get rid of its intrinsic characteristic properties.
Moreover, even an e-paper application, moving picture or video image reproduction is also somehow natural requirement in terms of required function as an e-reader. Under above requirements, entirely new types of power saving type displays with keeping good enough balance with memory type display's advantages are being required as an emerging technology.
Current so-called e-reader type display technologies are also expected to be applied digital signage type large bill board displays. As is well known, most of bill board types of large screen displays need specific illumination regardless self-emission and/or illumination system to enhance reflective nature of the screen. Although additional illumination system is required, a reflective display system keeps its specific advantage under bright enough luminance environment which is usual in fine mid-day in most of places worldwide. Of course, night time and very dark environment, more or less, specific illumination system is required. Even if such an illumination system is required, effective surface reflection of reflective base display gives more effective right reflection, resulting in significant power saving effect for large bill board types of display systems. Under current energy saving requirement situation in general, this better reflectivity is even effective for display systems required specific illumination systems.